Windows: transient parent is set to 0 when parent is about to be destroyed, breaking parent-child z ordering

I have three windows: w1, w2 and w3. w1 is the transient parent of w2, and w2 is the transient parent of w3. This means that w2 is always in front of w1, and w3 is always in front of w1 and w2.

When w2 is destroyed, the transient parent of w3 is set to 0, not w1 as one would expect. So w3 can now be hidden by w1.

In practice, this is an issue that sometimes arises in the context of dock widgets (I have a dock widget that creates a dialog; when the dock widget is docked on the main window, the dialog loses its transient parent and can be hidden by the main window.)

A small test case showing the problem is attached.